There are many different kinds of clarifier tanks for the treatment of unclarified sewage. Many round clarifier tanks have a center column made of concrete, with a center-driven mechanism, rotatable around the column, in the shape of a cage, to which a multiarmed bottom rake mechanism is attached. Rake blades are attached to the arms which are arranged within a certain distance from the bottom. These rake blades move the sludge, which settles at the bottom, to an area were it can be removed from the tank.
Because a part of the biomass in the clarifier tank does not settle at the bottom but rises to the water surface, many clarifier tanks have to be equipped with mechanisms to remove this scum. Known are rake mechanisms with scum rake blades, scum boxes and overflow weirs. Also known are so called skimmers to remove scum from the water surface. These are comprised of pipes with an opening on the side, through which the scum enters the pipe and is drained. It is also standard practice for the waste water inlet pipe to run underneath the tank into the column.
An opening on the side of the column allows the unclarified sewage to enter the clarifier tank.
According to new guidelines, clarifier tanks have to take on additional functions, such as e.g. the removal of phosphates, nitrification and denitrification. According to current guidelines, the total contents of solids in the drain of the secondary clarifier tank should be limited to 20 mg/l.
To satisfy these requirements, it proved to be practical to replace the usual overflow weirs used in common clarifier tanks with submerged effluent pipes projecting from the sidewall of the clarifier tank towards the center of the tank, with openings to drain the clarified sewage from the tank through the sidewall into a effluent circumferential launder surrounding the clarifier tank. By using these submerged effluent pipes, the clarified sewage is removed below the water surface on which the scum accumulates, and is drained out of the system into the effluent launder surrounding the tank.
In addition, the scum removal output of newer clarifier tanks should be increased and the scum be removed quickly, easily and as completely as possible, even if it is of a light creamy consistency.